In the case, for example, of automobile radiator caps the industry today requires pressure tolerances of 0.1 bar for the opening of the pressure/vacuum valve by the pressure or vacuum existing in the container. This requirement is difficult to obtain with normal springs since their tolerances in general lead to too great pressure tolerances.
The invention has as its object therefore the provision of a cap of the aforementioned type whereby in a simple way and without the use of special springs it can be achieved that the pressure/vacuum valve is opened by a pressure value lying within narrowly prescribed limits.
This object is solved in accordance with the invention in that the compression of the pressure spring is adjustable. Therefore the opening pressure of the pressure/vacuum valve can be exactly adjusted. The tolerances of the employed spring therefore play no role.
The adjustment of the compression of the pressure spring can be achieved, in a simple way, by the pressure spring being made from a helical spring which engages with its end remote from the sealing membrane a support body provided with threads and engaged with mating threads on the inner part so as to be adjustable in the axial direction of the pressure spring. Preferably the support body is formed as a ring with a U-shaped profile whose radially inner wall is received in the interior of the helical pressure spring and defines a polygonal opening and whose radially outer wall on its outer side carries the threads making it threadable into a correspondingly threaded cylindrical portion of the inner part. The radially inner wall can, for example, be formed as an hexagonal surface so that the support body can be easily turned by an hexagonal key inserted into the opening, and so that the compression of the spring can in this way be adjusted. The helical spring used as the pressure spring is ground at its end surfaces to provide flat bearing surfaces. To achieve a uniform distribution of the pressure of the pressure spring onto the flexible support disk, in accordance with the invention the pressure spring at its end turned toward the sealing membrane engages a plate supported on the flexible support disc by means of a ball joint. For such ball point the plate can have a ball shaped protrusion in its center mating with a corresponding ball shaped depression in the support disk.
Further features and advantages of the invention will be apparent from the claims and from the following description of the invention as explained in association with the accompanying drawing showing an exemplary embodiment.